not able to add a curve, the popup menu only lets me add meshes
Most likely you are trying to add a curve while editing a mesh in edit mode. Meshes and curves are different kind of objects and cannot be combined.
Curves can't be added to a mesh object, and meshes can't be added to a curve object.
Exit edit mode and you will be able to add a curve as a different object.
Blender has different kinds of objects, all of them have different and unique characteristics and cannot be combined into objects of a different kind:
Meshes
Curves
Surfaces
Metaballs
Text objects
Lattice
Since you are starting with blender you should take a few minutes and read through the very basics of the manual:
https://docs.blender.org/manual/en/latest/modeling/index.html
it will save you a lot of time down the line.